The present invention relates to a supply voltage controlled stereophonic amplifier system which has a high power efficiency and a simple circuit configuration.
In general, a stereophonic reproduction system has a wide dynamic range of processing signals. Therefore, in order to minimize the distortion of the power amplifier circuit due to excessive input signals, a relatively high power amplifier circuit is necessary.
However, the use of a class "A" amplification circuit causes troublesome problems resulting from much power consumption and heat dissipation. For this reason, a class "B" amplification circuit has been so far employed in many applications, even if a class "B" amplification output circuit requires a special circuit to prevent the switching distortion thereof.
FIG. 1 shows an example of such a class "B" amplification circuit which includes an input signal source 1, a pre-amplifier 2 for amplify the input signal to a predetermined output voltage level, output transistors 3 and 4 for power amplification, a loud speaker 5, and batteries 6 and 7.
As well known, the power efficiency (.eta.) of a class "B" amplifier is given by EQU .eta.=P.sub.o /(P.sub.o +P.sub.c)
Where, P.sub.o is an output power of the amplifier to be supplied to the associated lound speaker, and P.sub.c is a loss power. The power efficiency (.eta.) of a prior art class "B" amplifier varies with the output level (P.sub.o), as shown by curve A in FIG. 2, in which the abscissa represents the normalized output (P.sub.o /P.sub.omax) when the maximum output P.sub.omax is expressed by 100%, and the ordinate represents the power efficiency (.eta.). It will be seen from FIG. 2 that the power efficiency of the class "B" amplifier is good and 78% at the point of the maximum output, but for the smaller output levels, the power efficiency drops remarkably in such a manner that the average efficiency becomes less than 30%, for example, with respect to musical signals because the difference between the peak level and the average level is great. As a result, a use of higher power amplifier circuits involves increased heat dissipation, thereby making the heat radiation design difficult. In addition, in the case that the amplifier circuit is fabricated in the form of an IC circuit, this imposes serious limitation to the allowable maximum output of the IC circuit.
To eliminate such problems, there has been proposed by the same inventors as the present application in the Japanese Patent Laid-Open No. 73862-1976, an amplifier circuit in which the power supply comprises a switching DC-DC converter and the duty factor of the switching voltage is controlled (on the pulse width modulation (PWM) basis) according to the input signal to provide the variable supply voltage (which has been so far fixed), thereby to improve the power efficiency.
FIG. 3 shows an example of the disclosed amplifier circuit which has a d.c. power supply 8, a DC-DC converter 9, positive and negative output voltage terminals of the DC-DC converter, a full-wave rectifying circuit 12, a PWM converter 13, and a pulse amplifier 14. The d.c. power supply 8 consists of a.c. power input terminals 17, 18, diodes 19, 20, 21 and 22 for full-wave rectification, and a smoothing capacitor 23 and acts to rectify and smooth the a.c. supply voltage directly. The DC-DC converter 9 further includes a switching transistor 24, high-frequency transformers 25, 26, high-speed rectifying diodes 27, 28, and smoothing capacitors 29, 30. The PWM converter 13 includes a triangular or saw tooth carrier wave oscillator 15 and a comparator 16. The other reference numerals 1 to 7 in FIG. 3 represent the same or equivalent circuits or components as or to those in FIG. 1. It will be appreciated that the full-wave rectifying circuit 12 may be replaced with a half-wave rectifying circuit, in which case, the similar carrier converter and comparator must be added for the negative region of the input signal.
The comparator 16 compares the absolute value .vertline.e.sub.i .vertline. of an input signal with a carrier wave CR of, for example, 200 KHz, and if CR is less than .vertline.e.sub.i .vertline., then produces a pulse of constant level H. The pulse width W of the output wave PWM varies with the input signal .vertline.e.sub.i .vertline. (see FIG. 4 (B)).
In the operation of the circuit of FIG. 3, an input signal e.sub.i is supplied from an input signal source 1 via a pre-amplifier 2 to output transistors 3 and 4. The signal e.sub.i is also fed to the full-wave rectifying circuit 12 where the absolute value .vertline.e.sub.i .vertline. of the signal is sent to the pulse width modulation (PWM) converter 13. In the PWM converter 13, the absolute level .vertline.e.sub.i .vertline. is converted to the corresponding PWM wave to drive the DC-DC converter 9. That is, the driving of the DC-DC converter 9 with the PWM signal can allow the corresponding change of the output voltage .+-.e of the converter 9, i.e. the output voltage of a main-amplifier circuit MA including a pre-amplifier 2 and output transistors 3 and 4, according to the absolute value of the input signal e.sub.i.
The operation of the DC-DC converter 9 will be next explained. When a positive voltage is applied via the PWM converter 13 and the pulse amplifier 14 to the base of the switching transistor 24, the transistor 24 is turned on. This will cause an induction of a voltage on a primary winding of the high-frequency transformer 25, which voltage has the polarity shown in FIG. 3 and is equal to a voltage across the smoothing capacitor 23.
In this connection, a secondary winding of the transformer 25 is invertedly connected to the primary winding thereof so that a voltage on the secondary winding is inverse to a voltage on the primary winding with respect to the polarity. Therefore, the rectifying diodes 27 and 28 will provide an open circuit to the secondary winding, so that energy is accumulated in the primary winding of the high-frequency transformer 25.
Subsequently, if the switching transistor 24 is turned off, then the primary winding is opened so that the energy accumulated in the primary winding is supplied to the amplifier circuit MA via the rectifier diodes 27 and 28.
On the other hand, according to the theoretical analysis, the output voltage .+-.e which develops on one terminal 10 or 11 of the smoothing capacitor 29 or 30 while the diodes 27 and 28 are in the conduction state, is expressed as ##EQU1## where, D is the duty factor by which the switching transistor 24 remains turned on, N.sub.1 is the number of turns of the primary winding in the high-frequency transformer 25, N.sub.2 is the number of turns of the secondary winding in the high-frequency transformer 25, E.sub.1 is a voltage across the smoothing capacitor 23, and V.sub.F is the threshold voltage of the current diode 27 or 28.
It will be obvious from the above equation that a change of the duty factor of the switching voltage causes the corresponding change of the ratio of the number of turns of the primary winding with respect to that of the secondary winding, thereby providing the rectified outputs proportional to the duty factor with respect to the output terminals 10 and 11.
In other words, the driving of the switching transistor 24 according to the PWM signal converted from the input signal e.sub.i will produce the rectified outputs depending on the PWM signal, whereby the supply voltage of the amplifier circuit can be controlled according to the level of the input signal e.sub.i so that the output transistors 3 and 4 always operate substantially in their maximum output conditions.
There is shown the relationship between the supply voltage .+-.e (shown by broken lines) and the output voltage e.sub.o (shown by a solid line) of the main amplifier circuit MA in FIG. 5 in which the abscissa represents the time and the ordinate represents the voltage, E.sub.m and -E.sub.m are the peak and valley voltages of the output voltage e.sub.o, and E.sub.o and -E.sub.o are the minimum and maximum levels of the supply voltages +e and -e, respectively. In this way, a small constant voltage E.sub.o is applied at all times only to that one of the output transistors 3 and 4 which remains turned on. This means that the power loss of the arrangement of FIG. 3 is considerably reduced thereby to improve the power efficiency remarkably, when compared to a known one.
More specifically, when the output voltage of the transistor 3 in operation is E.sub.1 at a time t.sub.1, the supply voltage is very small and slightly E.sub.2. It will be appreciated that this results in remarkably reduced power consumption from the fact that a constant voltage E.sub.3 (which is greater than E.sub.2) is applied at all times in the case of a prior art power supply. At the same point of time t.sub.1, a negative voltage E.sub.4 is applied to the transistor 4. Since the transistor 4 is in the cut-off condition, however, the power loss of the transistor 4 is substantially zero.
Although the above explanation has been made in connection with FIG. 5 in which the d.c. power supply 8 is used to rectify and smooth a voltage directly from an a.c. power line (for example, from a commercial 50 Hz/60 Hz a.c. power source), it will be understood that the d.c. power supply may be replaced by a suitable d.c. source such as an automobile battery or dry cells, without providing a change of the basic operation thereof.
It will be also noted from curve B in FIG. 2 that the power supply controlled amplifier circuit of FIG. 3 is improved in its power efficiency from about 1/3 to about 1/2, especially the power efficiency for the smaller output signals is substantially increased.
Although the amplifier system of FIG. 3 is relatively complex in the circuit configuration; the system allows the use of the compact heat radiator due to the improved power efficiency and the use of the smaller power supply transformer because of its high frequency operation, in addition to a substantial reduction of the power consumption. This will allow an achievement of a lighter-weight and more compact system.
An audible or sound signal amplifier system generally is of stereophonic type, that is, has two input/output channels as a left channel and a right channel. In addition, although it is too much to say that there is no correlation between the right and left channel signals perfectly, in some applications, there may exist a substantially different correlation therebetween. However, on an average, it is common that there does not exist not so strong correlation therebetween. Therefore, where the amplifier system of FIG. 3 is employed in a stereophonic amplifier circuit, exactly the same system of FIG. 3 is required for each of the left and right channels, which requires a larger scale power supply circuit, thereby cancelling the advantage of the original smaller radiator and power supply transformer.